Arsenite oxidase from Alcaligenes faecalis is an 85 kDa protein induced by the presence of AsO2- (arsenite). It functions to oxidise AsIIIO2- to AsVO43- (arsenate), which, although it is a structural analogue of phosphate and inhibits phosphorylation by producing unstable arsenylated derivatives, is less toxic than AsO2-, which binds to essential sulfhydryl groups of proteins and dithiols such as glutaredoxin, Arsenite oxidase was isolated from A. faecalis by Anderson et al. at Ohio State University. Spectroscopic studies showed that the protein contains one molybdenum, five or six irons, and inorganic sulfide, where the molybdenum appears to be in the form of a molybdopterin and the irons one [4Fe-4S] HiPIP or [3Fe-4S] cluster and one Rieske-type [2Fe-2S] cluster. Together with the electron acceptors azurin and cytochrome c (E0;320 mV), arsenite oxidase provides a relatively high potential electron transfer system for detoxification of arsenite (E0;60 mV) to arsenate. We have successfully grown seven crystal forms (P1, P2, C2, I222, P4, P41212 and R32) from preparations of arsenite oxidase. Depending on the form, the limit of diffraction is between 3 E and approximately 1.2 E. We have collected data sets from the P2, I222, P4 and P41212 forms but have concentrated on the P1 form, which, although it is almost invariably twinned, is the most reproducible. We have collected data to 2.10 E from native P1 arsenite oxidase. Analysis of the self-rotation function and the plausible solvent content of the cell (a=91 E, b=110 E, c=118 E, a=98 , _=90 , and ?=96 ) indicated that the cell contains four unique molecules.